This invention relates to an apparatus and method for generating lobular polygonal forms for various purposes. The apparatus and method of this invention is particularly applicable to the machine tool art and is suitable for producing wire drawing and heading dies for use in the production of screw blanks for certain types of self-tapping screws.
One such well known type of self-tapping screw is shown and described in Phipard, Jr., U.S. Pat. No. 3,195,156. The screw of that patent is of the type having an arcuate polygonal cross sectional shape with an odd number of sides. In commercial practice this cross sectional configuration is characterized by three equally spaced lobes having a radius of curvature substantially less than one-half of the pitch diameter of the screw and with the lobes being separated by relatively broad arcuate sides each having a radius of curvature substantially greater than one-half the pitch diameter of the screw. The sides merge smoothly and continuously with the intervening lobes respectively. Self-tapping screws of this type are further characterized as having a substantially low driving torque and a relatively high stripping or failure torque. Such screws have enjoyed enormous commercial success throughout the world.
The aforesaid self-tapping screws are roll-threaded from a blank of preformed shape of lobular polygonal form, or alternatively having a tapered end of lobular polygonal form and a main body or holding section of circular cross section. In any event, wire shaping dies are used to reshape circular wire either throughout the length of the blank or at the tapered lead section into the trilobular form. Thus, a screw blank can be formed from stock by drawing a length of round wire through a forming or sizing die that has an arcuate polygonal, lobular, triangularly shaped orifice of a size and shape for producing the arcuate triangularly shaped lobular blank. Stock material so formed can then be fed into a conventional cold heading machine wherein predetermined length may be severed and the end portion of the length upset and headed by conventional upsetting and heading punches.
The manufacture of the wire drawing die and the heading die requires considerable time and skill in order to provide an accurate cross sectional shape. In this regard it should be noted that the lobular form is of a type that is of substantially uniform width throughout 360.degree.. By that it is meant that when the polygonal form in the aforesaid U.S. Pat. No. 3,195,196 is revolved about its center, the polygonal form will be capable of maintaining tangency with two parallel planes. Therefore, the lobular form is capable of being rolled between two parallel roll-threading dies; however, the motion of the blanks is somewhat irregular and die wear may be a problem.
If the wire drawing dies are inaccurate as to shape, the result will be an error in the cross section of the blanks. In the heading dies, the lobular blank is received within the lobular die cavity for retention while the heading punches cold work the end of the blank to form the screw head. Desirably, the cavity of the heading die should conform closely to the configuration of the blank in order to prevent galling of the die upon removal of the headed blank from the die. Galling reduces die life.
The wire drawing and heading dies are sometimes made utilizing slightly different methods resulting in slightly different cross sectional shapes, thus contributing to the aforesaid problems. The production of tooling for these dies first requires careful grinding of the long arcs or major radii one at a time to form a hob or an electrode. The minor or small radii constituting the lobes are normally hand polished which often result in a lack of uniformity. These inaccuracies are passed on to the blanks and, in turn, to the threaded fasteners.